1. Field of the Invention
Aspects of the present invention relate to a bipolar output voltage charge pump circuit and more particularly, they relate to a bipolar output voltage charge pump circuit which provides two bipolar output voltages, i.e. two pairs of opposite polarity output voltages.
2. Description of the Related Art
Bipolar, i.e. dual rail, output voltage charge pump circuits are a type of DC-DC converter that utilize transfer and storage capacitors as devices to respectively transfer and store energy such that the converter is able to provide, from a unipolar, i.e. single rail, input voltage source, a bipolar output voltage that may be different in value from that of the unipolar input voltage.
In use, single bipolar output voltage charge pump circuits may comprise two output storage capacitors, typically known as “reservoir capacitors” and one or more energy transfer capacitors, typically known as “flying capacitors”. The terminals or connectors of the two “reservoir capacitors” are permanently connected to respective output voltage terminals or nodes. In contrast, the terminals or connectors of the two “flying capacitors” are capable of being switched, in a controlled sequence, to input or output voltage terminals or nodes or to the other flying capacitor terminals or nodes.
For example, a known single bipolar output voltage charge pump circuit, as disclosed in the present applicants co-pending UK patent application GB 2444985, can provide positive and negative bipolar output voltages (+/−VV/2) that are each equal to half the magnitude of the charge pump circuit's unipolar input voltage.
Furthermore, by suitable control, the co-pending UK patent application can also provide positive and negative bipolar output voltages (+/−VV) that are each equal to the magnitude of the charge pump circuit's unipolar input voltage.
Such a known bipolar output voltage charge pump circuit uses an arrangement, i.e. a network, of switches to control the connection of the terminals of the two reservoir capacitors, i.e. the two output voltage terminals, and those of the flying capacitors. The flying capacitor terminals may be connected by these switches to: the input voltage terminal, i.e. the unipolar input voltage; the output voltage terminals, i.e. the bipolar output voltages; a reference terminal, e.g. ground potential; and to one another in order to obtain either the bipolar output voltage +/−VV/2 or +/−VV.
FIG. 1 schematically shows a known audio output chain 10, utilising a charge pump 12. The audio output chain 10 receives input audio signal data 14 and after processing and amplifying the audio signal, outputs an audio signal 15. Audio signal 15 may be output to a load (not illustrated) such as headphones, speakers or a line load, possibly via a connector (not illustrated) such as a mono or stereo jack.
Input audio signal data 14 is first processed in a digital processing block 16, which is powered by DVDD and DVSS, say 1.2V and ground i.e. 0V, giving output binary digital signals with output logic levels equal to DVDD and DVSS. These output logic levels are then level shifted by digital level shifter 18 to logic levels of VV and VG required to drive the digital-to-analog converter (DAC) 20, supplied by supplies VV and VG, say 1.8V and ground. The level shifted audio data is then converted to analog signal data by the DAC 20. The output from the DAC 20 is input to a first amplifier stage 22, and then onto a second amplifier stage 24 which may be a headphone amplifier.
In FIG. 1, the first amplifier stage 22 is powered by the input supply voltage VV and a reference voltage VG, say ground. To maximise signal swing in each polarity, the amplifier will be configured so that its output will preferably be biased approximately half way between VV and VG, e.g. at VV/2. However, if the second amplifier stage 24 was also powered by the input voltage VV and the reference voltage VG, the amplifier output voltage would again preferably be centred about VV/2. To avoid passing d.c. current though the load, e.g. a speaker with other terminal grounded, a coupling capacitor would be required in series between the amplifier output and the load. It is well known in the art that this series connected coupling capacitor needs to be a large value to allow adequate bass response, and so tends to be physically large and expensive. Also on power-up and power down charging this capacitor up to its quiescent voltage of VV/2 is liable to cause audible pops, clicks and other audio artefacts in the audio output signal 15. Techniques are known to reduce these artefacts, but in practice cannot remove them completely, and demands of users for reduced audio artefacts are becoming ever more stringent.
In order to eliminate the above problems, the circuit of FIG. 1 uses an analog level shift block 26, to level shift the output from the first amplifier stage 22 such that the DC quiescent voltage is removed and the signal out of the second amplifier stage 22 is balanced around zero volts i.e. ground. A charge pump circuit 12 (or some other bipolar supply means) is then necessary to provide from a unipolar supply VV, a bipolar supply voltage (VP, VN) to the second amplifier stage, to allow the second amplifier stage to drive the audio output signal 15 at either polarity centered about ground
As can be seen from FIG. 1, the charge pump circuit 12 receives an input voltage VV and a reference voltage VG, say ground, and is clocked by a clock signal CK. The charge pump circuit 12 also has a flying capacitor 28. The output voltage VP, VN of the charge pump 12 may be +/−α·VV, where α may be 1 or 0.5. In this way, as the audio output signal 15 from the second amplifier stage 24 may be balanced around the reference VG, in this case ground potential, the problems associated with having to have the large coupling capacitor therefore no longer exist.
However, in FIG. 1, it is necessary to perform an analog level shift on the output signal, centred on VV/2, of the amplifier 22 to bring its quiescent level down to ground. The analog level shifter 26 is shown as connected between the output of amplifier 22 and the input of output driver stage 24: in some implementations it may comprise a resistor network within driver amplifier stage 24, coupled to the output of the amplifier stage 24. This analog level shift is undesirable, as any shift from VV/2 to ground may lead to some voltage drop across some resistance, and hence power will be wasted. The level shift circuit itself may introduce audio artefacts on power-up.
Further, charge pump circuits, such as charge pump circuit 12 shown in FIG. 1, are widely used in portable electronics devices where decreasing power consumption in order to extend battery discharge time is becoming ever more important. For an audio chain driving a 16 ohm headphone for example, typical listening levels in a quiet environment may require only 100 μVV (40 mV rms or 2.5 mA rms for a 16 ohm headphone). However if this current is supplied from a +/−1.5V supply (required to drive 50 mW peaks for audibility in noisier environments) then the 2.5 mA rms sourced from the 1.5V supply consumes 3.3 mW, i.e. an efficiency of 100 μW/3.3 mW=3%. Even if the supply voltage (VP, VN) can be halved using the aforesaid known charge pump described above, then the efficiency is still poor, and reducing the power supply further makes it difficult practically to get enough voltage headroom for the input stage of amplifier 24.
Further, especially at low signal levels, the power required to switch the switching devices of the charge pump may be significant enough to degrade the efficiency.
Furthermore, in order to drive transducers such as piezoelectric transducers, haptic transducers or backlights for example, bipolar output voltages of greater than VV may be required. The same output chain may be required to drive such loads in some use cases, with a consequent requirement for operating modes with bipolar output stage supply voltages greater than VV.
It is desirable to be able to operate a particular charge pump circuit, particularly an integrated circuit implementation, in various applications which may have different supply voltages available. In order to maintain similar performance with different input supply voltages, it is desirable to have a range of step-down and step-up ratios available.
Charge pumps that generate a range of output voltages may have multiple flying capacitors. These flying capacitors are generally too large to be accommodated on-chip, so require dedicated pins on the package a well as occupying area on the PCB. It is desirable to minimize the number of flying capacitors to reduce cost, package size and board area.
It is therefore desirable to provide an audio output chain and an appropriate charge pump that can supply a wide range of output stage bipolar supply voltages to reduce or minimize power consumption over a wide range of output signal levels and input supplies while allowing adequate signal swing in the rest of the chain without requiring any analog level-shifting in the signal path, while providing a low cost and small physical size.